Pinhole detector testing means and method



Dec. 24, 1968 F. Y. MASSON PINHOLE DETECTOR TESTING MEANS AND METHODFiled Dec. 31, 1964 5 Sheets-Sheet 1 I 17 POSITION CONTROL MANUAL INPUTPROGRAM CONTROL ACTUATION LASER PULSE OR TO ACTUATE LASER FROM BEAM ANDBRBBB To PROVIDE AEBRRBS IBM CARD OF PERCED HOLES HOLE COUNT ACTUATEDLASER NUMBER OF PIERCED START END OF HOLES RUN SIGNAL DATA LOGGER NUMBEROF PIN HOLES DETECTED 25 PINHOLE 1234567891011 DETECTOR READOUT TAPE(START-END) FIG. I

FREDERICK Y. MASSW INVEN'IOR.

BY JJ. 0. flmw Ma. (HM

ATTORNEYS 1963 F. Y. MASSON 3,

PINHOLE DETECTOR TESTING MEANS AND METHOD Filed Dec. 31, 1964 5Sheets-Sheet 2 FREDERICK Y. MASSON INVENTOR.

ATTORNEYS Dec. 24, 1968 Filed Dec. 31, 1964 F. Y. MASSON 3,418,482

PINHOLE DETECTOR TESTING MEANS AND METHOI? 5 Sheets-Sheet 5 SERVO MOTORCONTROL CIRCUITRY FREDERICK Y. MASSON BY INVENTOR.

ATTORNEYS Dec. 24, 1968 F. Y. MASSON PINHOLE DETECTOR TESTING MEANS ANDMETHOD Filed Dec. 31, 1964 {A AAAAA/R/R v 5 Sheets-Shet 4 FIG. 4

FREDERICK. Y. M ASSON INVENTOR.

ATTORNEYS Dec. 24, 1968 F. Y. MASSON PINHOLE DETECTOR TESTING MEANS ANDMETHOD Filed Dec. 31, 1964 5 Sheets-Sheet 5 FREDRICK Y. MASSON INVENTOR.

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ATTORNEYS United States Patent 3,418,482 PINHOLE DETECTOR TESTING MEANSAND METHOD Frederick Y. Masson, West Orange, NJ assignor to GeneralPrecision Systems Inc., a corporation of Delaware Continuation-in-partof application Ser. No. 356,784,

Apr. 2, 1964. This application Dec. 31, 1964, Ser.

Claims. (Cl. 250-219) ABSTRACT OF THE DISCLOSURE A method and means forchecking the operating effectiveness, during normal operation of apinhole detector unit by intentionally making a predetermined number ofholes in the material to be inspected and determining whether theseholes are properly detected by the apparatus. The invention may beutilized in connection with either the inspection of essentially flatmaterial or material which is formed from flat stock into alongitudinally sealed tube, as in the manufacture of metal cans.

This application is a continuation-in-part application of the Joseph C.Scanlon and Frederick Y. Masson US. patent application Ser. No. 356,784filed Apr. 2, 1964.

The present invetnion relates to optical systems in genoral, and moreparticularly to methods and means for eliminating errors in suchsystems. As will become more fully apparent as this descriptionproceeds, the present invention relates specifically in itsapplicability to optical systems which include a radiation sensitivedetection means which responds to radiation of a particular wave lengthrange, a light source for exciting the radiation sensitive detectionmeans and a carrier medium having alternating transparent and opaquesections interposed between the light source and the detection means.Accordingly, by the way of example, but not limitation, the inventionwill be described herein as applied to a particular apparatus embodyingsuch an optical system, viz., a pinhole detector, the common term ofreference for devices which automatically inspect very thin sheets ofmetal, such as tin plate, or like material, for minute perforations.

The detection of pinholes is particularly important in the sale of sheetmetal which will be later used as the stock for making cans. In thisapplication, the stock from which cans are made, i.e., metal plate, issent to the tube mill which will produce the cans in the form of largerolls of elongated flat strips wound on pay-off reels. The fiat stripsare then unwound from the pay-off reel along a long table between amultiplicity of rollers usually referred to as squeeze rollers. Thesqueeze rollers gradually curve the strip laterally as it travelslongitudinally across the table so that it takes on a tubularconfiguration with an open longitudinal seam. At this point, the opentube is led past a welding station where the two edges of the strip arewelded together by means known in the art. The finished tube thentravels along until it reaches a cut-off station where the tube is cutinto can lengths. Afterwards, the bottoms are placed on the can inanother operation.

The forming of the strip into a tube and the cutting up of the tube intocans is carried out very rapidly at hundreds of feet per minute, and inthe case of certain stock, difiiculty is encountered in cutting off thetubes at the cutofi station. To facilitate the cut-off operation, it issometimes necessary to partly cut into the width of the stock so that,eifectively, the tubing is cut into can lengths even before it becomestubing. The out can, usually made of 3,418,482 Patented Dec. 24, 1968ice thin tin plates," usually from 8 to 20 mils in thickness must nothave pinholes. To prevent the making of cans with pinholes, twoinspections are required. First, the flat material shipped from thesupplier, long before it is formed into a can, must :be inspected.However, a second inspection is required of the formed cans to detectpinholes made during the manufacture. There are many reasons whymaterial, which is in perfect condition at the supplier, Will havepinholes when formed into cans. The hole can exist because of animperfect Weld. Also, holes may be caused by the action of the coolingfluid, e.g., Water, grease, etc., at the welding station. The rapidtemperature change as the strip is formed into cans can also causepinholes. With a rapidly moving production line, a thousand cans can becut before the first pinhole is discovered, which will mean untoldwaste, not only in material, but also in man hours which must be spentto inspect each can suspected of having holes.

It is, therefore, apparent that two inspection points are required. Thefirst inspection point should be at the supplier to insure that nodefective stock is shipped to the can-making tube mill. The tube millreceiving station may, of course, for its own protection have its ownfirst inspection point which would be substantially the same kind ofequipment as the supplier. The second inspection station appearsnecessary past the welding station and before the cut-oif station.However, as already explained, here the tubing may already be partly cutoff into cans to facilitate the work at the cut-off station. Also, thewelding station is usually quite near the cut-off station and there isconsiderable equipment in this vicinity for the purpose of welding thestrip into a tube, cooling, current directing impedance means, guiderollers, etc.

The present invention relates to an optical pinhole detection systemand, broadly, there are two embodiments to the system; one to inspectfiat stock before the welding station, and the other to inspect theformed tube which will be cut oif into cans. One of the particularproblems to which the present invention relates is to the testing of thedetector. Obviously, if the detection means are not functioningproperly, the result is Worse than if there are no detection means atall.

Therefore, an object of the present invention is to pro vide a testingarrangement for radiation sensitive detection means which willautomatically ascertain whether the detection means are functioningproperly.

Another object of the present invention is to provide pinhole detectionmeans particularly in can making and includes pinhole detection meansfor the fiat stock and for the formed tubing before it is cut into cans.

A further object is to provide a novel method of insuring the operatingeffectiveness of pinhole detector apparatus during operation thereof toinspect a traveling length of material for the presence of pinholes.

Other objects and advantages will become apparent from the followingdescription and the accompanying drawing in which:

FIG. 1 is a block diagram of the pinhole detector and the associatedtesting arrangement contemplated herein;

FIG. 2 provides a perspective view of some of the components of thetesting arrangement described in block diagram in FIG. 1;

FIG. 3 is a fragmentary sectional view, partially schematic of oneembodiment of a pinhole detector contemplated herein;

FIG. 4 depicts schematically an equalization and testing circuit forsome of the components used with the pinhole detector contemplatedherein;

FIG. 5 is a fragmentary perspective view, partially schematic of anotherembodiment of a pinhole detector contemplated herein; and,

FIG. 6 is a front view along the lines 6-6 of FIG. 5.

Before going into a detailed description of the invention, it ispreferable to first see what is to be accomplished and examine generallyhow the present inventive concept proceeds to carry out its mission.

As already known, a fiat strip is inspected for pinholes by passing thestrip between a light source and light sensitive detection means. Aslong as no light passes through the strip onto the detection means, nooutput signal is received signifying that there are no pinholes. It isat once apparent that two sources of error exist. Light will leak aroundthe edges, and the detection means may not be functioning properly. Ingeneral, the problem of leakage aroundthe edges is dealt with in theparent application, Ser. No. 356,784. This problem exists, of course,only with a fiat strip. With respect to formed tubing for cans, anotherlight leakage problem exists which is described later herein. Thepresent application relates mostly to the prevention of errorinformation when the detection means are either not functioning properlyor, since a plurality of such means are used, if they are functioningunequally. Since the system for detecting pinholes in a flat strip issomewhat less complicated than that for detecting pinholes in a formedtube, the system for detecting the pinholes in the flat strip will befirst described in connection with the error prevention means.

The testing of the detection means is illustrated in block diagram inFIG. 1 which could be an illustration of a suppliers testing stationbefore a roll of sheet metal is sent out to a customer. The testing isstarted either by a manual input pulse or by a signal from acomputercontrolled punched card containing the customers codeidentification and the test specification which the customer desires tobe met. This starting pulse is supplied by input pulse box 11. The inputpulse is fed to a program control 13 which controls the mechanism toform a number of test holes. The program control will vary with thewidth of the strip and in turn actuates several other components. Thedata logger 15 is actuated by the program control and will record thenumber of pierced holes at the start and at the end of the test run. Theposition control 17 will move test hole making means laterally acrossthe strip. This is actuated by the program control when it is on oneside of the strip and it gradually will move the test hole making meansover to the other side of the strip. The hole count means 19 will, uponactuation by the program control, in turn act on the test hole makingmeans, also at the same time supply a count to the data logger each timea test hole is made. When the input pulse is given to the programcontrol 13 by the input pulse box 11, the position control located onone side of the strip will start moving over to the other side of thestrip. Mounted on the position control 17 is the test hole making means,i.e., laser beam 21 with associated electronics. This laser beam isactuated periodically by the hole count means 19 which in turn dependson the output from program control 13. Depending on the width of thestrip and the test desired, the laser beam will place a number of holesin the passing strip. Each hole so made is recorded by a data logger 15on a strip of paper. After the laser beam in the path of travel of thestrip is the pinhole detector 25, such as described in the parentapplication, Ser. No. 356,784. This pinhole detector 25 likewiseprovides an output to the data logger 15 and again the number ofpinholes is recorded. There should then be at least the same amount ofpinholes detected by the pinhole detector 25 as were made by the testhole making means 21. There can, of course, be more holes than testholes if the tested piece is defective, but there should not be lessholes if the detection means in the pinhole detector 25 are functioningproperly. A lower hole count indicates that either one of the detectioncells is not functioning at all, that there is an unbalance in theoutput from the various cells, or that the radiation source in thepinhole detector is defective.

A portion of the arrangement described in block diagram in FIG. 1 isshown in perspective in FIG. 2. Here is shown the position controller 17which is in fact a motor with gearing (not shown) moving a rod 27 whichin turn acts on a carriage 29. Carriage 29 moves on a trolley 31 andtracks 33 which extend under the strip to be tested. A carriage arm 35carries a laser source 21 so disposed as to shoot a laser beam down onthe passing strip as it moves laterally across the strip. As alreadystated, the number of holes made by the laser beam depends on theprogram control. The laser source 21 includes a fore and aft edgesensing means 37 and 39. These fore and aft edge sensing means may bemechanical and thus require physical contact between the strip and thesensing means, or they may be photocell sensing means as described inthe parent application, Ser. No. 356,784. As the laser source 21 travelsacross the strip, an angular series of pinholes 41 are formed. The stripto be tested is fed to guide rollers 43 and thence to the pinholedetector 25 which is the subject of the aforesaid parent application,Ser. No. 356,784.

As has previously been pointed out, the output from the pinhole detector25 to the data logger should be equal to or greater than the number ofholes purposely formed by the laser source 21. If a lesser count isobtained, three possibilities are presented; either one of thephotomultiplier tube sensing means is not functioning, or else theoutput from the different photomultiplier tubes are suiticiently unequalas to provide an incorrect output, or the radiation source is notfunctioning. The problem may perhaps best be understood by reference toFIG. 3 showing the construction of pinhole detector 25.

The basic construction of pinhole detector 25 is conventional and,therefore, has been shown more or less schematically. It consists of ahousing made up of an upper section 42 and a lower section 44 disposedand supported in spaced relation by structure, not shown, to permitindeterminate lengths of sheet material 46 to traverse therebetween.

Housing sections 42 and 44 are elongated in a direction transverse tothe direction of movement of tin plate 46 (which is perpendicular to theplane of the paper in FIG. 3) and are substantially the same at eachend. Upper section 42, of inverted-trough-shaped configuration containsa source of ultraviolet light which, in the illustrated embodiment, is afluorescent tube 48. Lower housing section 44, also trough-shaped butdisposed in an upright attitude, contains a bank of photomultipliertubes 50a, 50b, 50c arranged in a single row at uniformly spacedintervals. Pairs of reflectors 51, diverging upwardly from eachphotomultiplier tube, funnel light to the respective tubes.

Overlying the bank of tubes and reflectors, and closing the top of lowerhousing section 44, is a plate 54 of Pyrex glass superposed on a plate56 of filter glass or the like adapted to transmit only the particularUV wavelength emitted by tube 48, e.g., about 3600 A. From the structurethus far described it will be seen that, as sheet material 46 is drawnthrough the apparatus, UV light from source 48 will pass through anypinholes in the material and, impinging on the underlyingphotomultiplier tube, will produce an output signal via conventionalcircuitry, not shown.

In order to accommodate tin plate stock of different nominal width andto compensate for width variations which are characteristic of rolledstrip, a shutter assembly 58 is provided to prevent UV from source 48passing around the edge of tin plate 46 and causing spurious signalsfrom the detector tubes.

Due to the irregular edge of the tin plate, and to avoid wear andprevent ruflling the edges of thin stock, shutter assemblies arenecessarily designed to operate without making physical contact with theedges of sheet 46. To this end, the shutter proper, 60, takes the formof an assemblage of bafile plates which is U-shaped when viewed insection along a plane perpendicular to tin plate 46, the edge of whichpasses between the legs of the U. Specifically, shutter 60 consists ofan upper baflie plate 62 and a lower baffle plate 64 separated by asuitable spacer, such as block 66, between the outer edges of theplates. These features have been described in greater detail in theaforementioned co-pending parent application, Serial No. 356,784.

The photomultiplier tubes depicted schematically in FIG. 3 areillustrated in greater detail in FIG. 4. If one of these tubes iscompletely defective, there is, of course, no alternative but to replaceit. However, quite often this is not the case and, furthermore, whenthis is not the case, the replacing of the tube causing the difficultywill often not solve the problem. As can be seen in FIG. 3, there is infact a bank of tubes and, although the output from these tubes does nothave to be identical, there should be sufficient uniformity in theiroutput to supply proper information to the data logger. As is well knownby tube manufacturers, even the supplying of tubes with identicalcharacteristics at the start does not solve the problem since with thepassage of time, there is a drift in the sensitivity of the varioustubes which quite often differs from tube to tube so that after a muchlonger time, some tubes will be almost like new while others will becompletely worthless. To adjust the tubes to provide a uniformsensitivity across the detector width, there is, therefore, provided, asshown in FIG. 4, a voltage divider adjustment 52 for the photomultipliertube 50. This adjustment has a plurality of positions and in eachposition a resistance is added in series to the tube. Thus, theadjustment is based on the strongest tube which is biased down to thestrength of the weakest. In addition, there is an open position which isextremely useful in checking out individual tubes for noise and highdark current. It is sometimes possible that one pinhole will provide anoutput signal on two tubes. This can occur through defects in thedetector apparatus other than the tubes. To test for this possibility,as well as to test for other defects, e.g., defects in the data loggeritself, one or more of the photomultipliers can be set on thepotentiometer adjustment 52 open position.

As already mentioned, the inspection of the sheet material will insurethat the material to be formed into cans is free of pinholes, but asecond inspection is required for those holes formed during the makingof the cans, e.g., by defects in the machinery. To do this, aninspection station is required between the welding zone and the cut-offzone of the apparatus. In FIG. 5, the strip 46 is shown after it hasbeen partly shaped into a tube. For convenience, the numerous squeezerollers are omitted and the table along which the strip tra'vels is notshown. The view shown starts just before the welding zone and endstowards the cut-off zone. Because of the difliculty encountered incutting off the cans, the strip to be formed into tubing has partiallybeen cut by cuts 68 into can lengths. These cuts extend only partiallyacross the width of the can since the can must still be welded along theedges 70, 72. This is accomplished by means known in the art, e.g., highfrequency A-C welding and, to this end, a pair of welding contacts 74engage the edges 70 and 72. The two opposed edges 70 and 72 are broughttogether at a weld point 76 in advance of the electrodes, the currentrunning along the edges 70, 72 to the weld point and fusing the twoedges into a longitudinal seam. Since there is a strong tendency for thecurrent to go around the back of the tube where it is not wanted and notneeded, impedance means 78 are inserted beneath the weld point. Sincethis is high frequency A-C, the current tends to follow the path ofleast reluctance as opposed to the path of least resistance for lowfrequency A-C and will thus travel along the edges, i.e., the path ofhigh resistance, but low reluctance instead of going around the back ofthe tube, namely, the path of low resistance, but high reluctance. Theseimpedance means 78 are held in place by holding means 80 passing throughthe gap in advance of the weld point down vertically into the tube andthen horizontally parallel to the tube axis, or even along the tubeaxis. In addition to the impedance means 78, the holding means also havemounted thereon cooling means, usually a spray arrangement, which willspray a cooling fluid on the tubing past the weld point. There is,therefore, a hollow tubing within the holding means or the holding means80 is itself the hollow tubing for the passage of a cooling fluid, e.g.,water.

In the case of certain greases, e.g., silicone grease, which might haveoptical properties and cause false readings, it is sometimes preferableto color the cooling fluid black. Usually, the cooling fluid isrecovered and recirculated so that after a while, it naturally tends tobecome black anyways, but some trouble may be expected at the start of arun if shiny new silicone grease is used as the cooling medium. Mountedon holding means 80 past the cooling means 82 are front and rearcylindrical masks 84 and 86. These masks are to prevent the penetrationof light into the pinhole detector. Between the front and rear masks isan ultraviolet light source 88. This light source 88 is in fact a strobelight and is not operated continuously otherwise a false signal would begiven every time a slit 68 passed before the light. To prevent such afalse signal, the ultraviolet light source operates only when the slits68 are over the mask; while the slits are passing before the lightsource 88 the light source gives forth no light. The actuation of thelight source 88 is accomplished by inductive trigger means 90. Thiscomprises generally a large number of turns of wire held around theformed tubing, one or more multiples of can lengths away from thepinhole detector. If there is room, it should be located in advance ofthe detector, but, sometimes, there will not be room there and it can belocated past the detector at the cost of one or two cans. Passingthrough the inductive trigger means 90 is an excitation current suppliedby the high frequency A-C power source and in parallel across theinductive trigger means 90 are trigger switch means 92 and high valueresistance means 94. Now, while the tubing without the slit is passingthrough the inductive trigger means 90, the inductive trigger meanspresent a large reluctance to the flow of current through the coils and,in effect, the coils act as choke coils. This forces the high frequencyA-C current through the parallel path of the high value resistors 94 andtrigger switch 92. However, as the slit 68 comes through the coils,there is an immediate elimination of a large part of the reluctance tothe flow of current through the coil so that, momentarily, the flow ofcurrent across the parallel path of switch 92 and resistors 94 dropstowards zero. This drop in current triggers off switch 92 which iscoupled to light source 88 by circuitry passing along the holding means80. In this way, the light source 88 will flash only at the instant thatthe slit passes through the inductive trigger means 90. As has been already explained, the flow of the high frequency A-C current at thewelding station is somewhat enhanced by the proper use of impedancemeans 78. The same principle is used at the inductive trigger station,except this time it is the conductance across the coil which must beenhanced so that the sharpness of the trigger can be improved by use ofconductance co'il means 78a disposed on the holding means. Thisconductance coil means 78 may consist of a simple circuit of only one ora few turns likewise fed by the high frequency A-C power source but outof phase with the excitation current through the inductive triggermeans. As is well known, like currents repel and unlike currents attractso that the flow of an excitation current in one phase through theconductance coil means will attract current of the opposite phasethrough the inductive trigger means. However, when the tubing and notthe slit is passing between the conductance means 78a and the triggermeans, then the tubing acts as a shield between the conductance meansand the inductive trigger means. The phase shift for the conductancemeans may be accomplished by means of a phase lagging circuit 96, e.g.,a capacitor, inductance coil arrangement.

Needless to say, associated with masks 84 and 86 within the tubing arecorresponding outer masking means '85 around the tubing. These are shownin FIG. 6, showing only masking means 85 associated with front mask 84.The outer masking means associated with rear mask 86 is not shown sincethe positioning of this masking means is identical with outer maskingmeans '85. The pinhole detector 26 is cylindrical and has radialreflectors 53 similar to reflectors 51 for the pinhole detector used fora flat strip. Overlying the bank of reflectors 53 and around the innercylindrical wall of detector 26 is a plate 55 of Pyrex glass superposedon a plate 57 of filter glass, or the like, adapted to transmit only theparticular UV wavelength emitted by the light source 88, e.g., about3600 A. The reflectors 53 define a plurality of zones and in each zoneis a photomultiplier tube 51a, 51b, etc. As the tube passes through theapparatus, UV light from source 88 will pass through any pinholes in thematerial and impinge on the surrounding photomultiplier tube to producean output signal to be recorded on the data logger. In testing thedetector at this station, the test holes are made not in the partlyformed cans, but in the strip of material, i.e., before the weldingstation, i.e., regardless of whether it is the flat strip orpartly-formed cans which are being tested for holes, the test holes aremade as in FIG. 2 on the flat strip.

In describing the foregoing invention, emphasis has been on theparticular inventive features of the present application, and thosecomponents well known in the prior art and purchased commercially havebeen functionally described. Thus, the laser beam used for making testholes can in practice be a Ratheon, Trion TRG ruby laser. The datalogger and its associated equipment, such as the printer, can beobtained from Hewlett-Packard or Berkley. Switching circuits responsiveto current changes are well known in the art.

It is to be observed, therefore, that the present invention provides fora pinhole detector which also has a self-testing system and comprisesradiation sensing means, including mounting means for mounting saidsensing means in the vicinity of the workpiece to be tested for pinholeson one side thereof and supplying an output signal each time a hole issensed; radiation source means to emit radiation, including mountingmeans for mounting said source means in the vicinity of the workpiece onthe other side thereof so that radiation from said source means willimpinge on said sensing means if it passes through a pinhole in saidworkpiece; hole making means for producing at least one test hole insaid workpiece; actuation means to actuate said hole making means andproduce an output signal each time said hole making means is actuated;data logging means recording each time a test hole is made by said holemaking means and each time an output signal is supplied by said sensingmeans; and, position control means responsive to said actuating means tomove said bole making means relative to said workpiece so that the testhole can be made in predetermined locations. In practice, the sensingmeans will have a plurality of individual sensors, and as the positioncontrol means moves the hole making means relative to the workpiece, theactuating means will actuate the hole making means a plurality of timeswhile supplying an output to the data logging means. The number ofoutput signals from the sensing means and recorded on the data loggingmeans then must be at least equal to the number of holes made by thehole making means. The .hole making means is preferably a laser beammoved laterally across the workpiece. The sensing means consists of abank of photomultiplier tubes, each tube having a voltage divideradjustment in series therewith to equalize the output from the varioustubes so that each tube supplies substantially the same output signal inresponse to the same amount of radiation impinging thereon. The pinholedetector may be used for the detection of pinholes in a flat stripworkpiece in which ease the sensing means and radiation source arelodged in housing sections elongated in a direction transverse to thedirection of movement of the workpiece, one housing section being on oneside, the other housing section being on the other side of saidworkpiece. The light source in one housing section is usually a tube ofultraviolet light, while the sensing means in the other housing sectionis usually a bank of photomultiplier tubes in a single row at uniformlyspaced intervals separated by reflectors \divenging towards theworkpiece. It is also possible to test formed tubing (cylindrical orrectangular) "for pinholes; in this case, the pinhole detector ispreferably so constructed that the radiation source is mounted withinthe formed tubing and the detection means is mounted outside around theformed tubing. In the event that the formed tubing is already partiallycut, front and rear masks are provided at the predetermined cut lengthson each side of the radiation source. The radiation source isstroboscopic and the source is actuated by inductive trigger meanscoiled around the formed tubing. This inductive trigger means has a highvalue resistance in parallel therewith, and a high frequency A-Cexcitation is required to operate the inductive trigger means, saidfrequency being of the order of one hundred cycles or higher. A feedbackloop is provided between the trigger means and the stroboscopic source.As the tubing passes through the trigger means coiled around the tubing,a high reluctance is offered to the passage of the high frequency A-Ccurrent so that the current passes through the parallel high resistance.As the cut passes through the trigger means coiled around the tubing,this high reluctance momentarily disappears and the current goes aroundthe coil rather than around the high resistance. Switch means are sodisposed in the circuit of the inductive trigger means and its parallelhigh resistance that this momentary passage of current through thetrigger means coil enables the switch and closes the strobe circuit.This enabling of the switch may be enhanced by providing a secondcircuit of a small number of turns of wire within the tubing incombination with inductance-capacitance means to offset the phase of thesecond circuit with. regard to the inductive trigger means circuit. Thissecond circuit, i.e., conductance coil means, being fed an excitationcurrent from the same high frequency A-C source as the inductive triggermeans. Since opposite phases attra'ct each other, the current flow inthe second circuit will induce the current in the inductive triggermeans circuit to flow through the coil instead of the high resistance.

An additional use of this inductive trigger means circuit is as a speedindicator of the speed of the passage of tubing through the pinholedetector station. This speed indication in advance of the cut-offstation may be used to provide information if the cut-01f mechanism isproperly timed at the cut-ofi station.

By the present inventive concept, the detector is tested under dynamicoperational conditions rather than under static conditions. This is ofparticular importance since when testing under static conditions, it isusually impossible to understand the cause of the pinholes.

While there has been described what at present is believed to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is aimed,therefore, to cover in the appended claims all such changes andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In apparatus for detecting the presence of pinholes in a travelinglength of thin material by means of photodetectors arranged on one sideof said material to generate a signal in response to light passingthrough a pinhole therein, means for simultaneously checking theoperating effectiveness of said apparatus, said means comprising, incombination:

(a) piercing means for intentionally making a predetermined number ofholes in said material;

(b) actuating means operable to cause said piercing means to make saidholes; and

(c) counting means indicating the number of signals generated by saidphotodetectors, thereby providing a basis for comparison of the numberof said signals generated with said predetermined number.

2. The invention according to claim 1 wherein a plurality of saidphotodetectors, laterally spaced across said material, are provided togenerate independent signals in accordance with the lateral position ofa pinhole in said material, and further including positioning means formoving said piercing means laterally across said material so that holesmay be made adjacent each of said photodetectors.

3. The invention according to claim 1 wherein said material comprises ametal stock which is initially in the form of a fiat web and is formedwhile traveling into a longitudinally sealed tube, said piercing meansbeing positioned to make said holes while said metal is in flat form,and further (a) a light source adapted to emit radiation to which saidphotodetectors are sensitive; and

(b) holding means constructed and arranged to support one of said lightsource and photodetectors inside said sealed tube and the otherimmediately adjacent on the outside of said tube.

4. The invention according to claim 3 wherein said metal includeslateral cuts extending through and partially across said metal atpredetermined intervals along the length thereof, and further including:

(a) strobe means adapted to turn said light source on and 01f in rapidsequence; and

(b) synchronizing means for so timing the cycle of said strobe meanswith respect to travel of said material that said light source is offwhenever one of said lateral cuts passes between said light source andsaid photodetectors.

5. The invention according to claim 4 wherein said light source ispositioned inside said tube and said holding means also support maskmeans constructed and arranged to prevent passage of light from saidsource through adjacent lateral cuts in said tube when said source isturned on.

6. The invention according to claim 4 wherein said synchronizing meanscomprise inductive trigger means actuated by passage of said lateralcuts in said material past a coil.

7. The method of inspecting a traveling length of material to detect thepresence of pinholes in said material while insuring the operatingefiectiveness of the detecting apparatus, said method comprising thesteps of:

(a) positioning a light source on one side and light sensing means onthe opposite side of said material at an inspection station, wherebysaid sensing means are actuated to prdouce a signal in response to lightfrom said source passing through a pinhole in said material;

(b) intentionally making a predetermined number of pinholes in saidmaterial before the latter passes said inspection station; and

(c) counting the number of signals produced by said sensing means afterthe portion of said material having said predetermined number ofpinholes passes said inspection station to determine whether or not saidnumber of signals equals or exceeds said predetermined number.

8. The invention according to claim 7 wherein said material comprises athin metal stock initially in the form of a flat web and formed whiletraveling into a longitudinally sealed tube, said pinholes are madewhile the metal is in flat form and said inspection station ispositioned after the metal is in tubular form.

9. The invention according to claim 8 and including the further stepsof:

(a) making lateral cuts extending through and partially across saidmetal at predetermined intervals along the length thereof before themetal passes said inspection station; and

(b) sequentially turning said light source on and off in synchronismwith movement of said metal so that said light source is off wheneverone of said lateral cuts passes between said light source and said lightsensing means.

10. The invention according to claim 9 wherein the step of making saidlateral cuts is performed while said metal is in flat form.

References Cited UNITED STATES PATENTS 2,947,876 8/1960 Larew 2502193,238,357 3/1966 Minka 2502l9 X 3,263,086 7/1966 Brosious et al. 2502193,265,855 8/1966 Norton 33194.5 X 3,335,283 8/1967 Gingras et a1. 250219OTHER REFERENCES Third International Satellite is up A Goddard- ManagedProgram, Goddard News; vol. 6, No. 10, April 6, 1964, pp. 4 and 5.

RALPH G. NILSON, Primary Examiner.

T. N. GRIGSBY, Assistant Examiner.

U.S. Cl. X.R.

